The preferred embodiment relates generally to an apparatus for actuating a friction element. More particularly it pertains to a piezoelectric actuator for use in an automatic transmission.
A conventional step-ratio automatic transmission system uses wet friction components, such as band brakes and plate clutches, for automatic shifting. Friction components couple, decouple or ground rotating gear elements to alter a torque path within gear sets, thereby achieving a desired level of torque multiplication between an engine and the driven wheels. When used as a part of a gasoline-electric hybrid powertrain, the transmission system may include a gear set that couples torque from the engine with that from an electrical machine.
A typical friction element is actuated hydraulically, requiring an engine-driven oil pump and a complex hydraulic pressure control system that typically includes a number of pressure and flow control valves. A powertrain control unit controls the engine and the hydraulic control system in a coordinated manner for automatic shifting. The engine-driven oil pump draws an appreciable amount of torque from the engine, lowering overall powertrain efficiency. Alternatively, friction components can be actuated mechanically or through other means. However, these means tend to be physically large in order to deliver torque capacity comparable to hydraulically-actuated systems.
New applications of automatic transmission technologies, such as a gasoline-electric hybrid powertrain, require small, packageable, highly efficient actuators as an alternative to conventional systems.